The present invention relates to the field of transmission mechanisms for use in automatic gearboxes, and more particularly relates to a method for controlling a particular sort of transmission mechanism which provides various speed stages by use of several selectively engagable power transmission systems each of which is equipped with a clutching mechanism including a one way clutch, and a transmission control device for performing said control method.
A transmission mechanism is already per se well known for providing various speed stages between a first rotating member and a second rotating member, which comprises several power transmission systems, each of which can selectively be engaged to provide transmission of rotational power at its own particular speed ratio (or typically any selectable one of a plurality of ratios) between the first and second members, and each of which has a clutching mechanism which either can be connected so as to thus transmit rotational power or can be disconnected so as not thus to transmit rotational power. When the engagement and disengagement of the various clutching mechanisms is appropriately controlled, and concurrently the selection of the various speed ratios of the various power transmission systems is controlled, which may be done automatically by a transmission control system of a per se well known type so as to provide entirely automatic operation, it is therefore possible to automatically shift between a plurality of speed stages and to provide both powered and engine braking operation of the transmission as a whole.
A troublesome disadvantage of this prior art type of transmission mechanism is that its operation requires close and accurate control of the timing of the engagement and disengagement of the various clutching mechanisms. In the case that the engagement of one of the clutching mechanisms occurs somewhat too late after the disengagement of the previously engaged transmission mechanism, then for a certain intermediate period the engine associated with the transmission will race, especially during a power on shift as during acceleration upshifting of the transmission or during a kick down type downshift, and this can damage various elements of the transmission mechanism; and on the other hand, in the caes that the engagement of one of the clutching mechanisms occurs somewhat too early before the disengagement of the previously engaged transmission mechanism is complete, then for a short time period very large torsional strains are liable to be put on various elements of the transmission, in consequence of such an attempt, effectively, to engage two different speed ratios at one time, and this also can damage various elements of the transmission mechanism. This necessity for close control of the timing of the engagement and disengagement of the various clutching mechanisms has presented a severe disadvantage with respect to the realization of an automatic transmission incorporating such a transmission mechanism, because such close timing control is very difficult to ensure, especially after the transmission has been used for a long period of time and its operational time constants have altered with usage. One problem that arises, in the case of a hydraulic type transmission control system, is that over a certain time period changes in the viscosity of the hydraulic fluid and changes in the size of the various orifices of the control system can cause timing difficulties. Good timing control is important in the case of a power on downshift of the transmission or a so called kick down, when the engine load is high, and snappy and accurate shifting of the speed stages is particularly important and difficult in such a case, to provide good transmission operational feeling. Poor responsiveness of the transmission during a power on downshift or a kick down type downshift amounts to the same thing as if the vehicle itself has poor acceleration characteristics, and this is an unacceptable state of affairs.
Further, in design of a transmission for a vehicle, mountability of the transmission is an important factor which is required to be taken account of. With a front engine front wheel drive type of vehicle, there is present the constructional difficulty that the amount of axial space available for mounting the transmission has been very limited, which limits the axial length of the transmission as a whole.
A further very much desirable characteristic of such a transmission mechanism is that it should provide engine braking for the vehicle, at least in some of the speed stages thereof. Smooth starting off from rest of the vehicle, both forwards and backwards, are also very important, and it should not be necessary for unduly delicate control of the clutching mechanisms to be exerted in order to obtain this effect. Power loss in the transmission should be minimized. Further, while satisfying the above mentioned conditions, it is a general design objective of such an automatic transmission mechanism to be as light and compact as possible.
There exists a copending patent application Ser. No. 06/465,762, invented by the same inventors as the present invention and assigned to the same assignee, in which there are disclosed various embodiments of a novel type of transmission mechanism, and in which there is proposed a transmission mechanism for a vehicle, for receiving input of rotational power from a power supplying member which rotates in a particular rotational direction and for outputting rotational power to a power receiving member, comprising: (a) a first member which is rotatably mounted and which receives supply of said rotational power from said power supplying member; (b) a second member which is rotatably mounted and which supplies rotational power to said power receiving member; and (c) a plurality of selectively engagable power transmission systems each being selectively individually connected between said first member and said second member for transmitting rotational power between said first member and said second member; (d) each one of said plurality of power transmission systems comprising a clutching mechanism which can be selectively controlled either to freewheel in both rotational directions and not to transmit power in either rotational direction or to provide direct transmission of rotational power in one rotational direction only and to freewheel in the other rotational direction, said one rotational direction being the one which is required for power transmission to be performed through said clutching mechanism, during the transmission of power from said power supplying member rotating in said particular rotational direction to said power receiving member via said one of said power transmission systems; (e) at least one of said clutching mechanisms being also selectively controllable so as to be locked up so as to transmit rotational power in both rotational directions.
According to such a structure, because each of the clutching mechanisms can be controlled to freewheel in the one rotational direction which is the opposite to the one which is required for power transmission to be performed through said clutching mechanism, if two of the power transmission systems relating to two different speed stages of the transmission are engaged at the same time, no difficulty will arise with relation to the transmission becoming locked up, because of the free wheeling capability of the lower speed stage. Further, since one of the clutching mechanisms can be selectively locked up so as to transmit rotational power in both rotational directions, engine braking will be available in the speed stages which are made available by the power transmission system associated with this clutching mechanism, which conveniently may include the first and the reverse speed stages.
A requirement has arisen for a control method for controlling such a type of transmission mechanism, and for a device to perform the method, and in particular such a method and such a device which can take good advantage of the construction of such a transmission mechanism in order to provide good and snappy shifting of the transmission between speed stages thereof, particularly during a kick down, as explained earlier.